feat: add process manager and execution store types (0007b, 0007c)

Process spawn/wait/kill functions for subprocess management with output
capture, timeout, and process group cleanup. DagRun and DagStepResult
types for SQLite execution persistence.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/infra/process_spawn.go

@@ -0,0 +1,74 @@
+package infra
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"syscall"
+	"time"
+)
+
+// ProcessSpawn launches a subprocess using the given shell.
+// If shell is empty, "sh" is used. If command contains newlines it is treated
+// as a multi-line script: the content is written to a temp file and executed
+// with `shell <tempfile>`. Otherwise it is executed with `shell -c <command>`.
+// dir sets the working directory (empty = inherit). env sets the environment
+// (nil = inherit parent env). The process group is created with Setpgid so
+// that ProcessKill can target the whole group.
+func ProcessSpawn(command string, dir string, env []string, shell string) (*ProcessHandle, error) {
+	if shell == "" {
+		shell = "sh"
+	}
+
+	var cmd *exec.Cmd
+
+	if strings.Contains(command, "\n") {
+		// Multi-line script: write to a temp file and execute it.
+		tmp, err := os.CreateTemp("", "fn-proc-*.sh")
+		if err != nil {
+			return nil, fmt.Errorf("process_spawn: create temp file: %w", err)
+		}
+		if _, err := tmp.WriteString(command); err != nil {
+			_ = os.Remove(tmp.Name())
+			return nil, fmt.Errorf("process_spawn: write temp file: %w", err)
+		}
+		if err := tmp.Close(); err != nil {
+			_ = os.Remove(tmp.Name())
+			return nil, fmt.Errorf("process_spawn: close temp file: %w", err)
+		}
+		cmd = exec.Command(shell, tmp.Name())
+	} else {
+		cmd = exec.Command(shell, "-c", command)
+	}
+
+	if dir != "" {
+		cmd.Dir = dir
+	}
+	if len(env) > 0 {
+		cmd.Env = env
+	}
+
+	// New process group so we can kill all children as a group.
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+
+	// Use buffers instead of pipes to avoid race between Wait() and ReadAll().
+	var stdoutBuf, stderrBuf bytes.Buffer
+	cmd.Stdout = &stdoutBuf
+	cmd.Stderr = &stderrBuf
+
+	start := time.Now()
+	if err := cmd.Start(); err != nil {
+		return nil, fmt.Errorf("process_spawn: start: %w", err)
+	}
+
+	return &ProcessHandle{
+		Cmd:       cmd,
+		Pid:       cmd.Process.Pid,
+		StartTime: start,
+		Dir:       dir,
+		stdout:    &stdoutBuf,
+		stderr:    &stderrBuf,
+	}, nil
+}